1. Field Of The Invention
This invention relates to computer systems and, more particularly, to methods and apparatus for providing a two conductor serial bus for communicating with the internal registers of computer circuitry arranged on a data bus in order to configure programmable chips of the computer circuitry.
2. History Of The Prior Art
As the physical size of circuit elements used in computer circuits grows smaller and the number of individual circuit elements in an integrated circuit grows larger, providing a sufficient number of terminals to connect the devices of the integrated circuit to external circuitry becomes a major problem. No matter how many terminals are provided, the number always seems insufficient to handle the signals needed. Along with the reduction in size of the area available for terminals due to the reduction in size of the integrated circuits has come an increase in the word size of the data handled by computers. Typically newer computers are being designed to handle thirty-two and even sixty-four bit word lengths. In order to provide paths for thirty-two bits of data, the number of conductors used by a typical bus approximates fifty. Consequently, it is very difficult to find room for terminals to transfer signals in addition to those transferred over a primary bus.
For this reason, it is often necessary to furnish signals to integrated circuits other than by the primary system bus. For example, if an integrated circuit of a computer system includes configurable registers, signals used for accessing those registers must be transferred to the integrated circuits in some manner. These signals often have very limited access to terminals on the integrated circuits, as has been pointed out, simply because of the reduction in the physical size of the circuits and the need for so many terminals to carry signals related to the primary system bus. This is especially true in a system in which the primary bus is expected to carry video information for display or other real time signals. Due to the width of the primary bus necessary to handle video signals, it is expensive and thus undesirable to multiplex different types of signals onto the primary bus and also to route the primary bus to several locations.
Consequently, it is very desirable to provide some alternative arrangement which uses a minimal number of terminals in order to transfer signals such as those signals used to access internal registers on an integrated circuit chip. Moreover, it is important that such a bus be able to accomplish signal transfer rapidly, accurately, and efficiently.